Stepladders provide a means for an individual to climb to a height for manipulating objects or performing work in locations where the stepladder must be free standing and not resting against a support. The stepladder depends solely upon its construction and erection to ensure its stability, in contrast to extension ladders that must be braced against a structure.
A typical stepladder is designed to be folded into a convenient size for storage and carrying, and the requirement that it be portable is a major design constraint, restricting the weight of the ladder. Thus, a typical stepladder will be found to weigh thirty pounds (approximately 15 kilograms) or less in weight, in order to ensure that it may be easily handled. It is also designed to be folded into a package that, insofar as possible, is not of significantly increased size over a folded ladder of similar height-reaching capability.
The basic A-frame design of the common stepladder as a self-supporting climbing device was conceived around the year 1880, and this design has been used with only minor modifications to the present day. It comprises a pair of vertical, elongated structural members or rails, between which are attached steps, and a complementary pair of rear legs which extend to the rear of the ladder from the top cap to form a supporting structure in the form of an A-shaped frame. Some modifications and improvements have been made to this basic design, but the ladder industry has followed this design without fundamental alteration to the present day.
The stability of stepladders is affected by the user's movement on the stepladder during use. The requirement that the stepladder be portable tends to reduce the weight to the minimum consistent with required structural strength. Since an average user will weigh about one hundred fifty pounds (about sixty eight kilograms), practically all the weight involved in the dynamic coupling between the stepladder and the user will be concentrated in the user, and there will be very little static weight tending to stabilize the ladder. Thus the location of the user's weight (the user's center of gravity) determines the static stability of the stepladder in use. If a plumb line from the user's center of gravity falls outside the footprint of the stepladder, then the stepladder will topple. Motion of the user on the stepladder will further couple to the stepladder, creating a tendency in the traditional double A-frame stepladder to “walk” over the surface. Such walking is indicative of one leg of the stepladder being unloaded and may result in toppling, which may cause potential injury to the user.
Since tripods are known to be inherently stable structures, especially on uneven ground, various attempts have been made to create stepladders of triangular structure, with independent rear legs. As an example, Harrison, U.S. Pat. No. 2,650,014 discloses such a structure. However, the independently articulated rear legs lack the firm bracing of the typical A-frame stepladder. The resulting structure may lack adequate dynamic stability.
A similar prior structure is shown in the inventor's prior published U.S. Pat. No. 4,754,845. The disclosure described a stepladder having a rigid step section means, a strengthening top cap, and two independently articulated and angled rear legs, the legs and step section means extending from the top cap to form essentially an equilateral triangular footprint. The articulation of the rear legs and step section means was accomplished by a triangular vertical column, or center post, extending downwardly from the bottom of the top cap and journaling a sliding coupler, or collar, to which rigid support means (rear braces) were attached each leg to extend and retract the rear legs as the collar was moved up and down along the vertical post. A rigid support means was also affixed to the step section means. The rear braces were linear and not hinged or broken as in the prior art.
While this invention was inherently stable because of its essentially triangular footprint, it had a number of problems. The sliding collar was prone to jamming with dirt because of its exposed position, and the pin locking the sliding collar in a particular location was vulnerable to shearing off and becoming lodged when weight was applied to the structure. Furthermore, the front and rear bracing attached to the sliding collar frequently caught the user's fingers as the stepladder was extended and retracted. It was found that, because of its placement near a step, the front bracing encouraged the user to use it as a support, which led to premature failure of the front bracing. The hinging arrangement of the step section means and rear legs to the top cap was weak and prone to breakage, as well as catching objects as the stepladder was extended and retracted. While the structure described in this patent was stable, it was not manufacturable or practical for commercial or home use.
The invention disclosed in International Publication Number WO92/11425 (Publication Date 9 Jul. 1992, based on PCT Appl. No. PCT/US90107498, to Baker, later abandoned) provided a number of improvements over this the disclosure of U.S. Pat. No. 4,754,845. One improvement consisted of a redesigned triangular center post and sliding collar. The locking mechanism was revised to eliminate a vertical slot and internal horizontal plate in favor of an external pin biased by a spring to externally engage holes vertically spaced along the center post. A lever controlling the release of the locking pin is recessed in channels on the sliding collar to prevent accidental release. Another improvement consisted of alignment channels on the sliding collar to maintain the braces in an equal angular position, reduce misalignment of the ladder, and maintain the entire structure as a more rigidly braced tripod. The resulting structure was substantially free from torsion or twisting moments induced either by placing the stepladder on an uneven surface or by the shifting of weight of the user (creating unequal loads on the supporting structure). However, this structure too exhibited a number of problems that rendered it impractical for use. The pin in the sliding collar was still prone to shearing and jamming as the user's weight was applied to the structure. Users were still prone to use the front bracing for support, resulting in premature failure of the front bracing.
Some industry enhancements addressing the stability issue involve stabilizing structures added to the basic A-frame support structure. Various methods have been proposed, including the implementation of a tapering rail configuration where the side rails are wider at the bottom than at the top. This method provides a wider base to the basic A-frame supporting structure but complicates the manufacturing process, in that the steps are not uniform in length. Other such methods include the use of spreader bars connected to the side rails. These typically pose a serious threat of injury to fingers and hands.
Another issue involves the materials from which the ladder is made. Originally, the A-frame supporting structure and all its accompanying attachments were made of wood. Wood is inexpensive but it is not durable, prone to rot and splits, and it is heavy. However, one advantage of wood is that when dry it does not conduct electricity or heat and is useful in applications involving electrical repairs. More recent A-frame stepladders have been fabricated of more contemporary materials such as fiberglass or metal. Each material has its own advantages and disadvantages. Aluminum is lighter and easier to handle, but conducts electricity. Fiberglass is non-conductive, tough, easy to form during the manufacturing process, but it tends to deteriorate in sunlight, exposing interior glass fibers to the hands. It is also brittle and relatively heavy, all of which create problems in carrying and handling. The Occupational Safety and Health Administration (OSHA) requires many employees working in an electrical environment to use a non-conductive ladder.
Other improvements have consisted of equipment supporting means for holding and securing work materials, such as paint buckets, tools, and the like, to the topmost portion of the stepladder for convenience and to reduce the number of trips the worker must make up and down the ladder in order to obtain additional equipment. Still other improvements have consisted of stabilizing means to prevent the stepladder from falling.
Ergonomics is the study of the relationship between individuals and their work or working environment, especially with regard to fitting jobs to the needs and abilities of workers. The essential nature of ergonomics is the convergence of the disciplines of human biology (especially anatomy, physiology, and psychology) on the problems of man at work. As applied to stepladders, ergonomics is an integral part of safety.
While these enhancements represent improvements over prior art, experience in field testing with prototypes has identified certain safety issues and ergonomic concerns that are not adequately addressed in the prior art. A prime concern is lack of shielding of hinge mechanisms. The exposed joints allow fingers to be pinched and collect debris on moving parts with a resulting potential for increased resistance to movement of the activating mechanism and rear legs, and excess wear at hinge points resulting in reduced reliability and potential structural failure.
By definition, a stepladder is a portable tool. Its justification depends on its usefulness. In normal use, a stepladder is hand carried to a job site, set up, and stood on to elevate the user while performing work. Ideally, then, in addition to having structural integrity and being reliable, a stepladder should be easy and comfortable to carry, simple and safe to set up, stable, and provide an adequate storage medium for hand tools and supplies. This description defines the meaning of functional ergonomics as applied to stepladders. Normally a stepladder is carried by holding it horizontally by the side rails. In materials other than wood, this is usually an open C-channel, with small-radius, or sharp corners. The weight of the average stepladder combined with this type of side rail makes it uncomfortable to carry. This same C-channel is the primary point of manual contact for most other handling of the stepladder as well.
The ultimate test of utility for a stepladder is its ability to store hand tools and supplies for the user while performing work. In the traditional stepladder, two surfaces have been given cursory attention toward this end, i.e. the paint tray and the top cap. Without restraining sidewalls, the paint tray provides little holding capacity. Any motion of the stepladder is telegraphed to this surface, and objects placed on it frequently end up on the ground. Holes in the top cap are limited mostly to screwdrivers and such. Together, they offer inadequate tool storage for the average user.
In everyday use, many stepladders are used in areas without a firm, level, supporting surface, e.g. a concrete floor. Particularly on construction sites, the ground is usually uneven, and frequently soft. An important safety consideration is the total area of contact of the ladder's feet with the ground, since this determines how well it supports the weight of the user. In the case of the traditional stepladder, the area of each foot approximates the cross-sectional area of the C-channels used for side rails and rear legs. This is inadequate to contribute meaningfully to the safety of the user in unimproved environments, particularly since the weight of the user is transferred from front to rear as the user climbs up the ladder, and the rear feet are normally much smaller.
Another prime concern, particularly in the inventor's prior tripod designs, is durability of the actuating mechanism. Extensive field testing revealed that, in the extended position, the braces connecting the sliding collar with the step assembly offer a convenient “platform” for the feet of the person standing on the stepladder, since these braces are positioned adjacent to a step. This puts heavy downward pressure on the actuating mechanism, and in particular, the sliding collar. This results in a significant shearing force on the locking pin holding the sliding collar in position along the center post. Since it is not possible to prevent the user from standing on these braces, the entire actuating mechanism, including the center post, is redesigned to provide structural integrity adequate to withstand such pressure.
As can be seen, there is a need for a safer, more functional, and ergonomic stepladder. Using the previous definition of ergonomics, the ideal stepladder should (1) have structural integrity, (2) be reliable, (3) be easily and comfortably carried, (4) be simple and safe to set up, (5) be stable, and (6) provide an adequate storage medium for hand tools and supplies.